The invention relates to a method and apparatus for controlling the radiation field intensity at low elevation angles for a phased array antenna, such as for mobile telephones in a satellite link system of communication, by control of the radiation pattern near the horizon of a phased array antenna, and more particularly for eliminating interference by reflected radiation from the surrounding terrain.
A mobile telephone system may utilize a geostationary satellite as a link between two points on earth that are too far apart for direct communication, or out of range of other systems now in place on earth. A satellite can relay large numbers of telephone communication signals between mobile units located within a wide area, such as the continental United States, using only a moderate bandwidth for communication.
At the mobile unit on the earth, a phased array antenna is mounted on top of a vehicle with the antenna tilted to produce a beam axis at an optimum elevation angle, typically .about.40.degree. for the central part of the United States. The antenna array is rotated to the optimum azimuth angle, either by rotation on the vehicle, or by reorientation of the vehicle on the ground. In either case, reflections from the surrounding terrain produce multipath disturbances. An object of this invention is to control the radiation intensity in the direction of the reflected radiation, thereby to reduce multipath disturbance or interference with the optimum antenna radiation pattern of the mobile unit.
Methods employed for control of the radiation pattern near the ground have involved using inductive gratings or absorbers. However, limitations in materials, size restrictions, and other technical objections require a new approach to eliminate multipath disturbances, and to maintain good circularly polarized axial ratios at low elevation angles. In the past, achieving good circularly polarized beams at low elevation angles has been extremely difficult.